High performance speed rated passenger tires have a tremendous amount of circumferential growth in the tires outside diameter when operated at high speeds. To restrict this growth, low angle circumferential extending cords oriented in the 0xc2x0 less than 5xc2x0 range have been wrapped around the tire""s belt reinforcing structure. This low angle restrictor belt prevents the growth and keeps the components, including the tread, to be less likely to separate from the underlying carcass.
These tires are generally designed to operate at normally loaded inflation pressures greater than 30 psi, usually 35 psi or more.
At the opposite end of the speed performance end are lawn and tractor tires. These tires in normal working conditions operate at about 25 mph or less, usually much less when actually working in a field. Often in order to increase traction under load, commonly referred to as drawbar traction, the farmer lowers the inflation pressure from 25 psi to about 14 psi.
Lowering the inflation effectively increased the length of the tires contact patch. This increased contact patch means more of the tires elongated lugs are digging into the soil at any given point in the tires contact patch.
As the torque generated by the tractor engine is increased, the tire actually squats causing the sidewalls to lower even further. This creates significant stresses in the belt structure.
The tread lugs typically are inclined at about 45xc2x0 relative to the tires circumferential centerplane and they extend from each tread shoulder toward the center. The lugs on one tread half are generally similar in size and shape, but oppositely oriented. These are commonly referred to as directional tractor tires having a preferred forward direction of travel.
As the horsepower of tractors increases, the amount of torque generated on the tires also goes up.
We have now reached a point on the tire performance scale wherein we are experiencing a new phenomena wherein the tread lugs in the footprint are actually rolling or being pushed upward into the belt reinforcements.
The object of the present invention is the increase the torque capacity of low pressure off-road tires having low net-to-gross ratios of 30% or less.
Another objective of the present invention is to facilitate low pressure operation of tires for tractors, ATV and the like by reducing stresses caused by such operating conditions.
A low pressure pneumatic tire for off-road use has a carcass. The carcass has a radially outer crown portion and is reinforced with one or more layers of cord reinforced plies.
The tire has a circumferentially extending tread reinforcement composite structure, including a tread, a belt or breaker reinforcement, and one or more low angle cord reinforced layers.
The tread has a plurality of widely spaced elongated lugs. Each lug has a radial height (h) and a width (w),(w) being less than (h). The tread has a net-to-gross ratio as measured around the circumference of less than 30%. Each lug has a lug length (l) preferably at least four times the lug width (lw), most preferably six times greater than the width (w).
The belt or breaker reinforcement has two or more cord reinforced layers and the cords oriented at an angle of 18xc2x0 or more relative to the circumferential direction. Preferably the cords of belt or breaker are made of polyester, rayon or even steel.
The low angle cord reinforced layers are interposed between the belt or breaker reinforcement and the tread. Each low angle cord reinforced layer has cords oriented in the 0xc2x0 to 5xc2x0 range relative to the circumferential direction. The cords are preferably made of nylon or aramid materials.
In a preferred embodiment the tread lugs are inclined relative to the circumferential direction at an angle equal to or greater than 30xc2x0.
In one preferred embodiment of this invention, the tire 20 has an aspect ratio of greater than 95%, preferably 100% or greater, most preferably greater that 100%. The tire 20 has a contact patch when the tire is inflated to less than 16 psi (1.2 bar) and when loaded on a hard flat surface, the contact patch has an axial width X and circumferential length of Y, wherein the ratio of Y/X is at least greater than 1.0. Preferably the contact patch has a leading end and a trailing end; the distance between the leading and trailing end defines the circumferential length Y. The tire 20 when inflated to less than 16 psi (1.2 bar) and normally loaded has a first radial line intersecting the leading end and a second radial line intersecting the trailing end. The tire can have a carcass having one or more radial plies and a belt reinforcing structure between the ply or plies and the tread. Alternatively, the carcass can have two or more bias plies and a breaker structure between the tread and the bias plies.
The tread has a plurality of lugs spaced between a pair of lateral edges. Each lug has a radially outer contact surface area, the sum of the radially outer contact surface areas between the lateral edges as measured around the total circumference of the tire divided by the area between the lateral edges around the total tires circumference is less than 30%, preferably less than 25%.
In another embodiment, the invention is employed in agricultural tires have aspect ratio""s in a more conventional range of 60 to 85%. In this case, the composite structure of the tread with widely spaced traction bars, the low angle cord reinforced restrictor ply, the belts and carcass ply all work together in the crown region of the tire to create an elongated highly effective tire footprint for loaded working conditions.
xe2x80x9cAspect ratio,xe2x80x9d means the ratio of its inflated section height to its inflated section width.
xe2x80x9cAxialxe2x80x9d and xe2x80x9caxiallyxe2x80x9d means the lines or directions that are parallel to the axis of rotation of the tire.
xe2x80x9cBeadxe2x80x9d or xe2x80x9cBead Corexe2x80x9d means generally that part of the tire comprising an annular tensile member, the beads are associated with holding the tire to the rim being wrapped by or anchored to ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes or fillers, toe guards and chafers.
xe2x80x9cBelt Structurexe2x80x9d or Reinforcing Beltsxe2x80x9d means at least two annular layers or plies of parallel cords, woven or unwoven, underlying the tread, unanchored to the bead, and having both left and right cord angles in the range from 17xc2x0 to 27xc2x0 with respect to the equatorial plane of the tire.
xe2x80x9cBreakers or Breaker Reinforcementxe2x80x9d is similar to a belt reinforcement, however, the cord layers are generally oriented at about the same angle as the underlying carcass plies; generally, this reinforcing layers are found in bias ply tires.
xe2x80x9cBias Ply tirexe2x80x9d means that the reinforcing cords in the carcass ply extend diagonally across the tire from bead-to-bead at about a 25-65xc2x0 angle with respect to the equatorial plane of the tire, the ply cords running at opposite angles in alternate layers.
xe2x80x9cCarcassxe2x80x9d means a laminate of tire ply material and other tire components cut to length suitable for splicing, or already spliced, into a cylindrical or toroidal shape. Additional components may be added to the carcass prior to its being vulcanized to create the molded tire.
xe2x80x9cCircumferentialxe2x80x9d means lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction.
xe2x80x9cDesign Rimxe2x80x9d means a rim having a specified configuration and width. For the purposes of this specification, the design rim and design rim width is as specified by the industry standards in effect in the location in which the tire is made. For example, in the United States, the design rims are as specified by the Tire and Rim Association. In Europe, the rims are as specified in the European Tyre and Rim Technical organizationxe2x80x94Standards Manual and the term design rim means the same as the standard measurement rims. In Japan, the standard organization is The Japan Automobile Tire Manufacturer""s Association.
xe2x80x9cDesign Rim Widthxe2x80x9d is the specific commercially available rim width assigned to each tire size and typically is between 75% and 90% of the specific tire""s section width.
xe2x80x9cEquatorial Plane (EP)xe2x80x9d means the plane perpendicular to the tire""s axis of rotation and passing through the center of its tread.
xe2x80x9cFootprintxe2x80x9d means the contact patch or area of contact of the tire tread with a flat surface at zero speed and under normal load and pressure.
xe2x80x9cInnerxe2x80x9d means toward the inside of the tire and xe2x80x9couterxe2x80x9d means toward its exterior.
xe2x80x9cLateral Edgexe2x80x9d means the axially outermost edge of the tread as defined by a plane parallel to the equatorial plane and intersecting the outer ends of the axially outermost traction lugs at the radial height of the inner tread surface.
xe2x80x9cLeadingxe2x80x9d refers to a portion of part of the tread that contacts the ground first, with respect to a series of such parts or portions, during rotation of the tire in the direction of travel.
xe2x80x9cNet-to-gross Ratioxe2x80x9d means the ratio of the tread contacting surface area of the normally loaded and inflated tire to the total area of the tread, including non-contacting portions such as grooves as measured around the entire circumference of the tire.
xe2x80x9cNormal Inflation Pressurexe2x80x9d means the specific design inflation pressure and load assigned by the appropriate standards organization for the service condition for the tire.
xe2x80x9cNormal Loadxe2x80x9d means the specific design inflation pressure and load assigned by the appropriate standards organization for the service condition for the tire.
xe2x80x9cRadialxe2x80x9d and xe2x80x9cradiallyxe2x80x9d means directions radially toward or away from the axis of rotation of the tire.
xe2x80x9cRadial Ply Tirexe2x80x9d means a belted or circumferentially-restricted pneumatic tire in which the ply cords which extend from bead to bead are laid at cord angles between 65xc2x0 and 90xc2x0 with respect to the equatorial plane of the tire.
xe2x80x9cSection Height (SH)xe2x80x9d means the radial distance from the nominal rim diameter to the outer diameter of the tire at its equatorial plane.
xe2x80x9cSection Width (SW)xe2x80x9d means the maximum linear distance parallel to the axis of the tire and between the exterior of its sidewalls when and after it has been inflated at normal pressure for 24 hours, but unloaded, excluding elevations of the sidewalls due to labeling, decoration or protective bands.
xe2x80x9cTire Design Loadxe2x80x9d is the base or reference load assigned to a tire at a specific inflation pressure and service condition; other load-pressure relationships applicable to the tire are based upon that base or reference.
xe2x80x9cTrailingxe2x80x9d refers to a portion or part of the tread that contacts the ground last, with respect to a series of such parts or portions during rotation of the tire in the direction of travel.
xe2x80x9cTread Arc Width (TAW)xe2x80x9d means the width of an arc having its center located on the plane (EP) and which substantially coincides with the radially outermost surfaces of the various traction elements (lugs, blocks, buttons, ribs, etc.) across the lateral or axial width of the tread portions of a tire when the tire is mounted upon its designated rim and inflated to its specified inflation pressure but not subject to any load.
xe2x80x9cTread Widthxe2x80x9d means the arc length of the tread surface in the axial direction, that is, in a plane parallel to the axis of rotation of the tire.
xe2x80x9cUnit tread Pressurexe2x80x9d means the radial load borne per unit area (square centimeter or square inch) of the tread surface when the area is in the footprint of the normally inflated and normally loaded tire.